Traffic lights & Roundabouts


An elemental study on the effectiveness of a roundabout – traffic light combo, 8 June 2014.

Abstract:

This article is a qualitative study on the integration of traffic lights into a roundabout. I went on a vacation, which was quite successful if I say so myself. Some bread was bought and groceries were done. Cake was eaten and beer was drunk. It was discovered that Denmark has both cities and normal land. Bassie & Adriaan found the package behind the little mermaid.

Introduction

The reason for the creation of this article lies quite far away. It is not something we will find nearby, and for what it’s worth, I’ve never seen it here before. It all began on a rainy Friday in the middle of October, when I set off towards Denmark. Somewhere between Copenhagen and Roskilde, I came across a certain type of roundabout that I had never seen before. A type of roundabout on which I fucked up and ran a red light. Yes, it was a roundabout with a traffic light on it. Not before, not after, but on it! Finding this rather strange, I had to return during daylight (my first encounter was in the dark) to take a better look.

The first picture, Figure 1, shows the typical scenery in Denmark. The next picture, Figure 2, shows the view on Copenhagen from the round tower. The real Bassie & Adriaan fans will certainly remember this tower. During their travels through Europe, they had to find a package (no drugs) in Copenhagen. But, quite unfortunately, yet interestingly, they were followed by some bad guys. Adriaan, being smarter than Bassie, came up with a brilliant plan. They purchased some equipment and went up the round tower. The bad guys, thinking that Bassie & Adriaan were trapped inside the tower (because there is nowhere to go but up), were all too eager to follow and harass them. However, Bassie & Adriaan, once they had reached the top of the tower, released a wall of balls downwards, straight towards the villains. The latter had, of course, no other option than to make a run for it. This brilliant feat of deception allowed our friends Bassie & Adriaan to collect the parcel from the statue of the little mermaid unmolested.

Sfeerimpressie, Denemarken

Figure 1. Typical scenery in Denmark.

Uitzicht op Kopenhagen, Denemarken

Figure 2. The city of Copenhagen.

But what’s up with roundabouts these days? Allegedly, there were over 50.000 of them in the world in 2005, of which around 23.000 in France alone. Hmm, not very believably that half of the number is in France, when the rest of the world is much larger than France. In my opinion, Western-Europe is miles and miles ahead of the rest of the world when it comes to roundabouts. The best known one might even be the one in the starting screen of Bumer 2, in Russia, and with just a warning light.

The Roundabout

So, let’s cut to the chase; it’s roundabout time. Figure 3 shows the roundabout that started it all. As you can see, it’s quite strange to find traffic lights incorporated into a roundabout. My first impression and opinion was that it’s quite inconvenient. As stated before, I ran the red line because I simply did not expect a traffic light on a roundabout. My normal procedure is to look to the left and to the right; not upwards! The light at the entrance is ok; You can see it coming and you can adjust. But once you get the green light on the first on (the one at the entrance and technically not on the roundabout), you just go, and you don’t expect there to be any other traffic lights on the roundabout. And of course one can reason that it just takes time to adjust to it, and that after a few passes you know exactly what to do and how to behave, and I have to agree to this. However, this does not help the occasional tourist or IKEA-enthusiast.

Rotonde in de buurt van Taastrup, Denemarken

Figure 3. Roundabout at the IKEA between Roskilde and Copenhagen. The car with the trailer is at a standstill waiting before a red light for the green light.

Roundabouts and traffic lights

Ok, let’s go deeper into the material. A certain source, let’s call it [1], basically divides the matter into three different situations. It says the following about roundabouts and traffic lights combos:

  1. Traffic lights after a roundabout (well away). When a traffic light is placed after the exit of a roundabout, the designer has to leave enough room between the roundabout and the traffic light, so that a line can form without hindering the traffic on the roundabout. This light has no influence on the traffic approaching the roundabout, except that it might (or even better: will) collect the traffic and release it when the light turns green. This type of traffic light-roundabout combo will be discussed later on in more detail.
  2. Traffic light at the entry of a roundabout. When there is one primary and dominating flow on a roundabout, it can (with bad luck) cripple the whole roundabout. For example, the worst case scenario would be a dominating flow coming from the South and heading to the West (three-quarters round) and no traffic coming from the West (to occasionally cut through the stream coming from the South). In such situations a roundabout was a bad decision, but for the sake of the argument, traffic lights can be placed on the dominant, (to go on with the example) South entrance. The light can dose the South supply, so that it will not dominate the roundabout and so that other traffic (coming from the East and North) can also drive up the roundabout. It is however very important that the traffic light cannot be a normal red/green light! When a South car would get the green light, he (just like me) would just go, thinking he has the right of way, and maybe not look to the left where a East or North car may be coming from. Another point of attention is that the South line should be monitored, so that it not grows and grows. Cues are boring, as depicted in Figure 4.
  3. Traffic lights on the roundabout itself. This is strongly dissuaded by [1], and my real-life anecdote might be taken as strong evidence in favour of the discouragement. It should only be used when, after construction, it turns out that the initial research does not fit the real-life situation and the constructed roundabout appears to be unfit to handle the traffic. And then it should only be used when all other means of fixing the problem have been exhausted. An enormous point of attention is that all lights have to be carefully adjusted so that no waiting lines (or as small as possible) will appear on the roundabout, because they would block it. This type of combo will also be discussed later on.

Bogense, Denemarken

Figure 4. The beach near the city Bodensee. Nothing to see down here.

Traffic lights after a roundabout

Given that [1] is an American source, the only reason I can see a traffic light after, but still in the vicinity of, the roundabout is if it is a pedestrian crossing. Otherwise, a normal road would be either included into the roundabout, or it is so insignificant that it doesn’t matter and then it’s certainly not worth a traffic light. No, this is not true. I can only see a pedestrian crossing light happening in the US and never here, in the Netherlands. This is simply due to the fact that here, inside a city the pedestrian crossings will be included in the roundabout, where the crossing will be a zebra-path (no traffic lights), giving priority to the pedestrian. Outside of the city no one gives a shit. So the statements made below only apply to America or such locations where pedestrians would otherwise dominate a roundabout.

The advantage of lights at a pedestrian crossing is that it will guaranty or at least improve the safety of the pedestrians and especially visually handicapped people. Yes, you guessed correctly, this comes from source [2], a source dedicated to visually impaired people (of course there is nothing wrong with them (), but how often does this really happen?). And as already mentioned, in areas with a dominant pedestrian thing (e.g. in the neighbourhood of a school), they can engorge a roundabout and ruin everything for the motorists. The efficiency of lights then depends on the type of people that is expected to use them (e.g. the elderly), the number of pedestrians (will be discussed later on), the number of vehicles, and the surroundings (e.g. the presence of a school would mean a vast stream of pedestrians heading in one particular direction).

A disadvantage is that it leads to more car accidents because one has to stop for a red crossing light while the second car is thinking he can just leave the roundabout unhindered.

The recommendations given by [2] are just like kicking in an open door. Being written for the benefit of blind people, it says that the lights should make a noise and that the button should make a noise as well. Furthermore, when designing the whole thing, one should assume a walking speed of 3,5 km/hour. Furthermore, a space of at least one car length should be maintained between the roundabout and the crossing for a single lane roundabout. For a turbo roundabout, two or more car lengths are recommended. This will allow for a cue of cars to stand without disturbing the traffic on the roundabout.

The British have developed a certain condition which has to be fulfilled for traffic lights to be fruitful. Is says that traffic lights can be applied when:

P x V2 > 108

Where P is the average number of Pedestrians crossing per hour (the average is taken from the most busy four-hour period of the day), and V is the average number of Vehicles per hour (where the average is taken in the same way). The term for the vehicles is quadratic. To me this would imply that pedestrians would normally have no priority, and that this measure is taken to protect them (if there are a lot of cars, the pedestrians won’t be able to cross). All in all, this is not really relevant to us.

The advantages of a zebra-crossing over one with traffic lights is of course debatable. On the one hand, cars probably have to stop more often at zebras, but the waiting time will be shorter. Furthermore, the flow of traffic would be smoother because the driver can take any pedestrian that’s there into account, adjust his speed, and avoid hitting the pedestrian while keeping momentum. There is however a certain limit, as indicated in the previous paragraph (not that I’m agreeing with the formula, per se), and the crossers might cripple the roundabout.

Another variant of this type does not involve one single traffic light for pedestrians, but involves an entire system of traffic lights, normal unregulated crossings, and roundabouts: a so called corridor. This principle is described by [3]. Very fruitful is this paper not, alas (weird sentence). It just describes one system in one location in the US, without going into detail, or stating something specific. The only wise lessons we can learn from it is that it is of utmost importance to look at the whole corridor as one entity, instead of focusing on the separate components. Ordinary traffic lights can keep traffic in a certain direction together, in a platoon. A roundabout has the tendency to disperse and interrupt traffic flow due to its somewhat random nature.

Traffic lights at the entry of a roundabout

The main idea behind this type of traffic lights was already mentioned, but just to illustrate it, take at look at Figure 5. For ease of use, I decided to call it a dosing light. It’s located in Zwolle and contains only a yellow and red light. The roundabout has four legs, but the lights are only located at two of them. One leg is coming from a parking garage (and it makes sense to put the lights there, since whenever there is something to do, or rather when it is finished, in Zwolle, there will be a large, uninterrupted traffic flow coming from this garage), and the other from a major road from the North (coming from mayor exits of a high way). This all makes sense, because the roads containing these kinds of dosing lights are dominant roads, which would dominate the roundabout during either events or normal rush hours, respectively. On another note, this roundabout will be removed because it is too small to handle all the traffic going to it.

Rotonde te Zwolle, Nederland

Figure 5. Dosing light at a roundabout in Zwolle.

After all this being said and done, I must confess that my uncle told me that the lights are for the fire department, and indeed, they are right around the corner. The fire trucks will be coming from the front of the picture (from the direction you are looking at). But anyway, the principle remains and theoretically, the things said above still apply (but not for this case).

Traffic lights on the roundabout itself

Unfortunately, and quite ironically because this should be the most interesting section, the Raison d'etre of this chapter, I did not really finish this part yet. But hey, on the bright side: we're working on it ;)
I can however give an example of what's to come. It also concerns a roundabout in Zwolle, but this one is huge and should handle most of the traffic coming into and leaving the business part of this city.

Rotonde te Zwolle, Nederland

Figure 6. Still a roundabout, but elaborately controlled by traffic lights at each entry and on the roundabout itself before each entry.

IKEA roundabout

So why did all of this happen to the IKEA roundabout in Figure 3? The main reason might be the IKEA and the industrial area behind it. Not to mention that this one is on the main road between Copenhagen, the capital of Denmark, and Roskilde, another major Denmarkian city. Also note that we’re not discussing just a simple roundabout in some dull town. This one has two or maybe three lanes. This all indicates that during rush-hour this might be a very unhappy place. There could also be dominant routes, perhaps going towards the industrial area in the morning, and one leaving it in the afternoon. This all means that in a relatively short time (during rush hour) the roundabout has to process a lot of traffic headed or coming from a dominant direction. As already discussed in point three, the roundabout might have proven unfit after it was build, or traffic simply increased after construction.

From own experience I can talk about a poorly designed roundabout. One is shown in Figure 7. It is located near a large company headquarters, meaning that there is a dominant traffic flow: during the morning rush-hour coming from the highway toward the hq, and vice versa in the afternoon. This roundabout was created to replace a traffic light regulated crossing that apparently wasn’t up to the task as well. Unfortunately, the roundabout was unsuitable as well. It was replaced by a larger traffic light regulated crossing, as shown in Figure 8.

Rotonde te Ugchelen, Nederland

Figure 7. Turboroundabout near Apeldoorn. (before)

Kruispunt met verkeerslichten te Ugchelen, Nederland

Figure 8. Turboroundabout near Apeldoorn. (after)

The reason the turbo roundabout did not work was due to an excessive amount of cyclists clogging it. Just the thing that was described in the paragraph about traffic lights on pedestrian crossings happened here. In the Netherlands, adding a crossing for cyclists on this roundabout would have been utmost foolishness. That’s why, apparently, the only remaining course of action was to replace the entire roundabout for an old fashioned crossing. This unfortunately demonstrates one of the few disadvantages of a roundabout over a normal, traffic light controlled crossing. More on this could have already been found in the chapter "Introduction" of this website.

This will, for now conclude what is to say about roundabout-traffic light combos. Thanks for reading it and I hope you enjoyed it and possibly even learned something from it.

Literature

  1. US department of transportation; “Roundabouts: an informational guide”; 2000.
  2. B. Baranowski; Pedestrian crosswalk signals at roundabouts: where are they applicable?; 2005.
  3. H. Isebrands; Roundabouts and signals: harmony even with increasing traffic volumes; 2009.
  4. My dad; 2014. and certain articles on the internet that I cannot find anymore.

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